U.S. Pat. No. 4,651,267, which is assigned to the Assignee of the present invention, discloses a proportional base drive circuit for a transistorized bridge inverter. The proportional base drive circuit disclosed in the '267 patent has a positive feedback circuit which applies current, proportional to load current being switched by the bipolar transistors, to the bases of the bipolar transistors.
One form of the proportional base drive of the '267 patent is illustrated in FIG. 1. The proportional base is achieved by a current transformer 20 which has a primary winding 20a coupled to the output of a common point of the bipolar transistor switches T1 and T2 for developing a voltage in the secondary windings 20b and 20c, respectively, across resistors R1 and R2 which is proportional to the load current. The current in the secondary windings 20b and 20c is respectively full-wave rectified by full-wave rectifiers 24 and 26 which are respectively comprised of diodes D3-D6 and D7-D10. The potentials which are switched by the bipolar transistor switches T1 and T2 are respectively from voltage sources E1 and E2 which are connected together at neutral N. Base drive logic 22 controls the conductivity of feedback switches S1 and S2 which when conductive permit positive feedback from the secondary windings 20b and 20c to be respectively applied to bipolar transistor switches T1 and T2 to control the degree of saturation as a function of load current.
The proportional base drive provided in the inverter of the '267 patent is advantageous for the reason that the operating frequency of the power bipolar transistors in the inverter may be increased by controlling the degree of saturation of the transistors as a function of the load current. The proportional base drive provided to each of the bases of the bipolar transistor switches T1 and T2 in the inverter is chosen to have a magnitude which permits the rapid turnoff of the bipolar transistors. The proportional base drive circuit insures that the base drive applied to the bipolar transistors is not so large as to drive the transistors so far into saturation for light loads that extra time is required to turn the bipolar transistors off under light load conditions as a consequence of excessive minority carriers being present in the base region.
While the proportional base drive circuit disclosed in the '267 patent provides a controllable base drive for achieving rapid turnoff, it has a disadvantage that current flowing in the freewheeling diodes D1 and D2, which are poled in antiparallel across the outputs of the bipolar transistor switches, causes a rectified current to be produced by the full-wave rectifiers 24 and 26 which applies positive base drive to the bipolar transistor switches T1 and T2 in a reverse conduction mode when the transistors in fact should be totally turned off. The reverse conduction mode is caused by the freewheeling diodes D1 and D2 being forward biased by a lagging current (lagging power factor). In the reverse conduction mode the emitter functions as a collector and the collector functions as an emitter with positive feedback from the freewheeling diodes D1 and D2 causing the transistor to be driven harder into the reverse conduction mode. The aforementioned full-wave rectification results in the bipolar transistor switches T1 and T2 being driven at least partially into the reverse conduction mode which represents an undesired mode of conduction which represents a power loss during current flow in the freewheeling diodes D1 and D2.